The process of obtaining a clear liquid from a highly turbid liquid is an important process in the food industry and in drainage treatment. As a method of removing suspended substances contained in a highly turbid liquid from the liquid, centrifugation is known.
As a centrifuge to be used in the centrifugation, a screw decanter-type centrifuge is known. However, although this centrifuge is known to be excellent from the viewpoint of the processing speed and scale-up, its centrifugal force is limited to only about 2000 to 3000 G so that efficient removal of suspended components is difficult. Another example of the centrifuge is a De Laval-type centrifuge, which is a high-speed continuous centrifuge with a centrifugal force of about 8000 G. However, treatment at a high solids concentration is impossible with this centrifuge. Moreover, particulate components cannot be completely removed by this centrifuge and they partially remain, which is problematic. Still another example of the centrifuge is a Sharples-type ultra-high-speed centrifuge with a centrifugal force of about 20,000 G. This centrifuge employs batch-type discharge, and the size of the centrifuge cannot be easily increased because of limitation of the strength, which is problematic.
Another effective method of increasing clarity of a highly turbid liquid is filtration. In the food industry, drainage treatment and the like, there are methods in which a highly turbid liquid of interest is filtered using a filter aid through a suction filtration device such as a precoat filter or through a pressure filtration device such as a filter press. Known examples of such methods include those using diatomaceous earth or pearlite, especially diatomaceous earth, as a filter aid (see Derek B. Purchas, Mompei Shirato, “Solid-liquid Separation Equipment Scale-up,” Gihodo Shuppan Co., Ltd. (1979)). However, it has been proposed that filter aids using diatomaceous earth may cause problems in disposal of used diatomaceous earth and safety of the diatomaceous earth itself (see Toshiro Murase/Eikichiro Aktasuka/Masato Shibata, “Solid-liquid Separation,” Korin Publishing Co., Ltd. (1988)).
Filter aids using organic substances such as cellulose have been developed to solve these problems. However, in treatment of a highly turbid liquid, a cellulose-derived filter aid is less effective in increasing clarity of the filtrate than diatomaceous earth, and use of a cellulose-derived filter aid results in a lower filtration rate, which is problematic
To solve these problems, a method in which the particle size distribution of a cellulose powder is controlled to use the cellulose powder as a filter aid (see JP 9-173728 A), and a method in which the content of cellulose in the filter aid is controlled (see JP 58-40145 A) have been proposed. However, in those methods, clarity of the filtrate and the filtration rate are insufficient for filtration of the highly turbid liquid, and the performances of these methods are poorer than the performance of diatomaceous earth.
Thus, to obtain a clear liquid from a highly turbid liquid, it could be helpful to provide a cellulose-based filter aid having higher performance to remove suspended substances than conventional filter aids, and a method of producing it.